1. Field of the Invention
The invention relates to coated powder material for producing three-dimensional bodies (3D bodies) by means of layer constituting methods (powder-based generative rapid prototyping processes), in particular by means of 3D binder printing, the production of 3D bodies therefrom, and the production and use of sintered bodies obtainable therefrom. Materials and methods of this type are known from DE 44 40 397 C2, DE 698 07 236 T2, DE 696 07 975 T2, DE 690 31 808 T2 and U.S. Pat. No. 6,335,097 B1.
2. Related Art of the Invention
Among the more recent and particularly interesting powder-based generative rapid prototyping (RP) processes is the 3D binder printing process.
In a first variant of 3D binder printing, a layer of particles or granules is discharged onto an underlying surface and moistened with a binding liquid in predetermined regions, which respectively correspond to a layer of an object to be generated. The particles are wetted and adhesively bonded by the binder liquid in the moistened regions. Subsequent evaporation of the solvents in the binder liquid has the effect that the particles adhere directly to one another, fusing together at least in their edge regions. 3D binder printing processes that are related in particular to this type are known for example from European patents EP 0 644 809 B1, EP 0 686 067 B1 and European patent application EP 1 099 534 A2.
In a further variant of this method, use is made of a binder liquid containing sintering aids which, when the moistened regions are dried, leaves behind sintering aids which make it possible for the particles to be sintered as a solid mass in a subsequent sintering treatment. Hard and compact sintered bodies can be obtained in this way.
EP 0 925 169 B1 discloses a further variant of the 3D binder printing process in which mixtures of particles, fillers and adhesives are used. The binder liquid is substantially formed just by a solvent for the adhesive contained in the mixture. The adhesive may in this case also be in the form of a coating of the particles.
A further process of generative rapid prototyping (RP) provides for the particles to be made to solidify in the layer by means of light, or laser-induced sintering (hereafter laser sintering). For this purpose, the layer is heated in defined regions by means of exposure to light, if appropriate also through a mask, or laser radiation to the extent that the particles can fuse together or sinter together.
The known generative RP binder printing processes have the disadvantage in the production of sintered bodies that the finished green compact or sintered body exhibits a distinct shrinkage in comparison with the region originally activated by means of binder liquid, light or laser light. This is attributable to effects during the activation and during the sintering, sintering being understood as meaning both the laser-induced processes during the hardening of the layer and the production of sintered bodies from the green compacts obtainable by the RP processes.
In binder printing, the particles move closer together when they are moistened, under the effect of the capillary forces and surface tension, in particular whenever adhesives that may be present are made to swell or dissolved by the binder liquid, so that a distinct shrinkage has taken place already during the shaping or after the drying to form the green compact. Shrinkage processes also occur in the case of exposure to light or laser sintering.
The effect of sintering, in particular with regard to the production of sintered bodies, can be divided into a number of stages. In the initial or early stage, the original particles are still visible. The first cohesion between the powder particles is created by bridge formation and grain growth. A slight shrinkage occurs. In the middle stage, the formation of a contiguous pore space takes place. The individual particles are no longer identifiable and the sintering shrinkage begins. In the late stage there is the reduction of the pore space, which becomes increasingly accessible from the outside (closed pores). In the limiting case, complete compaction takes place (dense sintering).
The main elements of the shrinkage are attributable to the middle stage.
The known coatings have the disadvantage that they are hydrophilic and therefore absorb water from the surroundings, in particular from the atmospheric moisture. This leads on the one hand to an agglomeration of the particles and on the other hand to a high binding liquid absorption in the case of 3D binder printing processes. This has a negative influence on the distinctness of image of the 3D binder printing process, the material shrinkage under exposure to the binder liquid, the storage stability, and also the handling of the powder.